Microscopic Theory of Second Harmonic Generation at Si(100) Surfaces

We apply a microscopic formulation to calculate the second harmonic spectra of clean and hydrogenated Si(100) surfaces. We find a trend of the surface-allowed E₁ resonance as a function of the hydrogen coverage, which is consistent with experiment. Another resonance at lower energy, recently observed at the clean surface, is also reproduced and explained in terms of transitions across surface states. Its quenching is clearly demonstrated as a function of hydrogen coverage. Our results for the energetically more favorable c(4×2) structure are in better agreement with low temperature experimental data than those calculated for the 2×1 reconstruction. We find that the spectra are dominated by the χ_∥∥⊥^s component of the second-order nonlinear surface susceptibility.